The present invention relates in general to a multilayer screen and in particular to a multiple ply screen for use particularly in a web forming part of a paper making machine. The screen is of the type which includes an at least two ply fabric made of longitudinal and transverse filamentary elements whereby the top side of the uppermost ply represents the upper side of the screen, and the lower side of the lowermost ply represents the lower side of the screen. The plies of the fabric are tied together at different tie points.
A dual wire dewatering screen of this kind is known from U.S. Pat. No. 3,127,308 where an upper ply of the screen fabric (outer wire loop) of relatively finer mesh woven in a plain weave, is connected with a lower ply (inner wire loop) of relatively coarse construction in a three-twill configuration (as a shute twill). The thickness of cross-section of the filaments in the lower wire loop or ply is greater than that of the filaments in the outer loop or upper ply. The upper side of the outer screen loop represents the forming wire on which a sheet or web fleece is made from the applied stuff suspension. The lower side of the inner wire loop forms the lower side of the run through which the dewatering screen engages processing parts of the paper making machine such as rollers, rolls, suction and draining devices past which the stuff suspension is carried by the circulating screen. In this known screen, the bind of the inner loop to the upper loop of the screen fabric is effected by several longitudinal or warp wires of the inner or lower fabric loop by guiding the latter in the outer or upper loop and interweaving the same with transverse or shute wires.
In this dual wire dewatering screen, the longitudinal wires of the lower screen loop form knuckles which pass under two transverse or shute wires. The warp wire knuckles thus are subject to tension present in the longitudinal direction of the sieve that is in the direction of a circulation and also to an increased wear or abrasion due to the contact with the before mentioned parts of the paper making machine. Both the tension and the wear negatively affect the service life of the screen. On the paper web forming side of this prior art dual wire apparatus, the tie points of the coarser web wires are raised above the forming wire and form projections. Due to the application of relatively thick warp or longitudinal filaments or wires in the upper ply of the screen texture, the uniformity of the structure of the upper side of the screen is disturbed and wire marks are reproduced in the mating surface of the paper web. The interference occurs in two respects, namely due to the presence of the filament or wire tie points by themselves which produce the before-described undesired projections on the paper web forming upper surface of the screen and secondly, due to the absence of warp tie points in the lower ply of the screen fabric there results weak or missing tie points in the weave. Because of these missing points the overall stability of the screen fabric and its strength are impaired. In addition a different dewatering affect takes place at these weak or missing tie points which can lead to the so-called hydraulic marking in the paper web.
From the German Gebrauchsmuster 7,438,850 a multi-layer or multi-ply dewatering screen is known having a relatively fine mesh and dense upper ply and relatively coarse-mesh and open lower ply and if desired, having additional intermediate plies of the screen fabric, which are bound together, by means of special, separate warp wires or filaments which are not a component of the weave of the screen fabric. These binding warp wires or filaments enclose at the tie-up points in the upper and lower plies of the screen fabric always one shute wire or filament.
In another known multi-ply dewatering screen (DE-OS 2,917,694) which also consists of a complete fine mesh upper ply and a complete coarse meshed lower ply of the screen fabric, for tying up the two plies there are employed separate shute tying wires or fibers instead of the separate warp tying wires. These separate shute binding filaments are linked so as to keep the position of the tie points from the outer sides of the sieve toward the inner sides of the latter. Inasmuch as the thickness or the cross-section of the binding wires or filaments are always less than the thickness or cross-section of the remaining warp and shute wires or filaments of the dewatering sieve, the raised tie points on the upper surface of the screen and thus the resulting disadvantageous indentation points in the paper web on the forming side of the screen, are avoided. The uniformity of the structure of the forming upper surface of the upper ply of the screen nevertheless is still subject to disturbances even in this prior art multi-ply dewatering screen due to the tie points of this additional binding wires or filaments. These tie points produce disadvantagous effects such as non-uniform retention and dewatering over the entire surface of the paper web during its formation as well as a non-uniform structure or markings on the web engaging the forming side of the sieve. Limits are set to the reduction of the number of the disturbing tying points because of the ensuing impairment of the stability of the multilayer dewatering screen. The possibility to reduce the thickness or the diameter of the binding wires or filaments is also limited inasmuch as at a given strength of material of the screen the reduction of the thickness of the binding filaments would produce an impaired binding between the plies of the screen fabric. In dewatering screens which consist of several, mutually independent complete fabric layers inter-connected by separate filaments, that means by binding filaments which do not belong to the woven fabric of the screen, particularly the binding filaments which extend traversely to the running direction of the screen are exposed to considerable shearing forces. This traverse load applied to relatively thin binding filaments leads frequently to breakage of the latter and the layers of the screen fabric become separated and the replacement of the entire dewatering screen becomes necessary. It has been also found that particularly in the dewatering screens of the type in which the binding filaments extend traversely to the running direction of the screen, a comparatively low resistivity against formation of waves in the screen occurs which also leads to frequent interferences or the necesssity to exchange the screen.
Known is also a double-layer dewatering screen (DE-OS 3,036,409) which avoids separate binding filaments and the disadvantages occuring in a double-layer dewatering screen fabric and which is constructed of an upper and lower layers shute or traverse filaments as well as of a layer of warp or longitudinal filaments; the shute filaments of both shute layers are superposed in pairs and the warp filaments are mostly interwoven in the upper layer and only a portion of the warp filaments is additionally tied to the lower shute layer whose shute filaments are preferably of a larger thickness than the warp filaments. According to the construction of the screen fabric in question, a single layer fabric is reinforced by a shute, namely by the so-called under shute system. It distinguishes therefore from the aforementioned multi-layer dewatering screens which have at least one shute filament- and one warp filament system per one layer. It is true that in this one-layer fabric reinforced by the under shute system the desired doubled-sided fabric is obtained within certain limits, nevertheless in view of warp filaments determining a marking free structure of the upper side of the screen, certain limitations with regard to the construction of the lower side of the screen cannot be avoided.